Load control devices, such as switches, for example, use electrical relays to switch alternating currents being supplied to an electrical load. The life time of such electrical relays may be shortened by arcs or sparks caused at the instant when the relay closes. Some prior art systems seek to suppress arcs by controlling the relay actuation time such that the relay contact(s) close as nearly as possible to a zero cross of the alternating-current (AC) waveform.
FIG. 1 depicts an AC voltage waveform as controlled by an example prior art relay switch control circuit. Waveform 100 depicts the waveform of the AC power source, where the portion in dashed line may represent the voltage of the AC power source, and the portion in solid line may represent the voltage across an electrical load. As shown, the waveform 100 may cross through zero volts at voltage zero crossings such as the zero crossings 110A and 110B. The example prior art relay switch control circuit may include a voltage zero crossing detector for detecting the zero crossings such as the zero crossing 110A. The example prior art relay switch control circuit may store a relay-actuation delay 120, which corresponds to the time interval between the relay actuation time and the time when the relay contact(s) initially close in response to actuation. In operation, the relay switch control circuit may actuate the relay at relay actuation time 130A prior to the next zero crossing 110B. As shown, the relay actuation time 130A leads the next zero crossing 110B, or the target zero crossing for relay closure, by the relay-actuation delay 120 such that the relay contact(s) close at a time corresponding to the target zero crossing 110B.
In operation, the example prior art relay switch control circuit detects the zero crossing 110A, waits for a relay actuation adjustment 150A, and actuates the relay at time 130A. The relay actuation adjustment time period 150A corresponds to the difference between a full AC cycle and the relay-actuation delay time period 120. When the relay contact(s) are closed at the zero crossing 110B, substantially no current flows through the relay contact(s). The value of the relay-actuation delay time period 120 may be updated to account for any variation caused by temperature, and/or aging or deterioration over the life time of the relay.
When a relay closes, however, there is a settling time before the relay contact(s) come to rest in the closed state. For example, as shown in FIG. 1, the relay contact(s) may bounce one or more times for a time period 140 before becoming steadily closed. Bouncing results in wasted energy that may dissipate in the relay contact(s) as heat. This heat may cause the relay contact(s) to weld and become inoperative.
Some prior art systems seek to address this problem by offsetting the relay actuation time by one-half of the relay contact-bounce duration. FIG. 2 depicts an AC waveform as controlled by an example prior art relay switch control circuit with bounce compensation. Here, the relay actuation adjustment time period 150B corresponds to the difference between a full AC line cycle and the sum of relay-actuation delay time period 120 and one-half of the relay contact-bounce duration 140. In other words, the relay actuation adjustment time period 150B is less than the relay actuation adjustment time period 150A by one-half of the relay contact-bounce duration. A relay actuation time 130B leads the target zero crossing for relay closure by the relay-actuation delay time period 120 plus one-half of the relay contact-bounce duration 140. Consequently, as shown in FIG. 2, the relay contact(s) may continue bouncing for a period right after a zero cross possibly during high current conditions, thus suffering from similar behavior as shown in FIG. 1. Relay bouncing during this time period may cause the relay contact(s) to weld. Further, in operation, the duration of the relay bounce period may vary with each closure of the relay, thus the relay may actually become steadily closed at any time within the relay contact-bounce duration 140.
Some prior art systems also control the relay open actuation time such that the relay contact(s) open as nearly as possible to a zero crossing of the AC waveform. The relay actuation time is offset by an open time delay in a time-aligned manner relative to a zero-crossing. The hope is that the relay contact(s) will actually be opened when the power source current is substantially zero amps. Such prior art systems check whether the open time delay is outdated due to hardware aging, and replace the present value with a new value upon detecting that the open time delay is no longer correct. This type of reactive correction may still result in relays opening with a high voltage. Unfortunately, when a relay opens with a high voltage, undesirable arcing may occur and may persist through the next zero crossing. This may significantly shorten the operative life of the relay.